fix check for in-project-venv

[dimbleby]

looks like #7792 was a bit careless, confusing two things

• it rightly replaced two "is in-project-venv enabled?" checks with "is in-project-venv enabled, OR not explicitly configured but the directory exists?"
• it wrongly replaced two "is in-project-venv enabled AND the directory already exists?" checks in the same way

this undoes those mistakes

fixes #7957

[radoering]

Is a test feasible?

[dimbleby]

unclear how to test.

test_activate_with_in_project_setting_does_not_fail_if_no_venvs_dir() executes the code path but the reason that it shows no problem is that the mocking check_output_wrapper() does not know that python -c ... is trying to call a non-existent python.

[manugarri]

maybe a way to test this would be to have an end to end test? create a new poetry environment with the flag set to true?

[github-actions]

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